1. Field
This invention relates to measurement of Distortion Product Otoacoustic Emissions (DPOAE), acoustic signals generated in the cochlea. In particular, it relates to a method to reduce DPOAE fine structure by using frequency modulated primary tones.
2. State of the Art
The mammal and human ear features an amplification system, which amplifies soft sound by up to 40 dB. This “cochlear amplifier” employs so-called outer hair cells (OHC) located in the organ of Corti in the inner ear. The mechanical activity of the OHC is non-linear, causing non-linear distortion to be produced in the inner ear.
The non-linearity of the ear has been known over a century, but it was relatively recent that the OHC of the cochlea were identified as the primary cause. The middle ear is quite linear over sound pressures of 40 to 110 dB SPL, and does not result in noticeable distortion at normal listening levels. The inner ear non-linearity does produce distortion, which can be heard, and measured in the ear canal. Measurement of distortion products in the ear canal is used as a hearing test for newborn infants, since the distortion products are absent for certain forms of hearing impairment.
In principle the ear non-linearity could be expressed as a power series; that is, where the response is linearly proportional to the sound pressure, plus a term proportional to the square of the sound pressure, cube of the pressure, etc. If two frequencies were present, a square term would produce intermodulation products equal to the sum and difference of the two frequencies. A cubic term would produce products equal to twice one frequency plus and minus the other frequency.
If continuous sinusoidal sounds are applied as a stimulus, the sound response generated by the inner ear can be separated from the stimulus by analysing the outside sound response generated frequencies of the stimuli. Since intermodulation products of the primary frequencies are generated as a result of the non-linearity of the functioning inner ear, the presence of signals whose frequencies do not match (“clash”) with the stimulus signals is a deciding factor in proving the integrity of the inner ear. These signals are termed distortion product otoacoustic emissions (DPOAE).
DPOAE are acoustic signals, generated in the cochlea of mammals, especially humans, as a response of two sine tones of different frequency (“primary tones”) used as stimuli. The probe for recording DPOAE typically contains two loudspeakers for stimulation and one or more microphones for recording.
Typically, primary tones with levels L1, L2 and frequencies f2 are set with f1<f2 and L1≧L2 and the frequency ratio f2/f1 is in the order of 1.2 to 1.3.
The strongest DPOAE component is generated at fDPOAE=2*f1−f2. This component is the one used by virtually all commercial DPOAE equipment.
DPOAE measurements work in a broad frequency range, from less then 500 Hz to more than 10 kHz, depending on the subject, recording equipment, and noise conditions.
DPOAE are thought to be generated by the so-called “outer hair cells”, which act as an acoustic amplifier in the cochlea.
Since the DPOAE are only a side effect of this cochlear amplifier, the signal that can be recorded in the ear canal is normally small in amplitude, compared to background noise and stimuli. This makes signal processing necessary to detect the signal combined with background noise.
DPOAE Detection
The common method to detect DPOAE is framing the measurement of raw data in frames of constant length, with all frequencies (f1, f2, fDPOAE being multiples of the frame rate).
The frames or the FFT components of the frames are usually averaged with some artifact rejection scheme to finally decide a DPOAE has been found or to determine its amplitude.
A widely used method to decide a DPOAE has been recorded is to observe the amplitude of the recorded spectrum at fDPOAE compared to the neighboring frequency components (“SNR criterion”).
In order to support the stimuli described below, DPOAE detection needs to be designed differently, to allow small, continuous frequency deviations.
DPOAE Fine Structure
When measuring DPOAE with a high frequency resolution, such as one measurement every 20 Hz, fine structure can be observed in most subjects. Fine structure in this context means, that the amplitude of DPOAE varies with frequency, and can show variations of up to 20 dB within as little as 100 Hz primary tone frequency modification. This fine structure is thought to be the result of outer hair cells in the regions that are tuned to the DPOAE frequency that generate otoacoustic emissions (OAE) themselves, which can constructively or destructively interfere with the original source, located at the overlap region of the primary tones f1 and f2.
The main aspects of DPOAE fine structure are outlined in the article, “Separation Anxiety: DPOAE Components Refuse to be Apart”, by Sumit Dhar, www.otoemissions.org/guest_editorials/2009/dhar/2009.htm.
An example of such a recording, ranging from 2 kHz to 4 kHz with 20 Hz resolution, is shown in FIG. 1. Three recordings were performed to prove the reproducibility of the fine structure.
DPOAE fine structure is unwanted in many applications of DPOAE measurements. In newborn hearing screening, test time is crucial. Hitting a fine structure minimum with one or more of the test frequencies can extend test time dramatically, or lead to refer results. Typically 4 to 6 frequencies are tested, with an “overall criterion” that passes a subject if, for example, DPOAE are found at 3 of the 4 frequencies.
When using DPOAE to estimate the hearing threshold, usually by extrapolating input-output functions, the fine structure, because it is level dependent, can severely corrupt the growth behavior at certain frequencies. This leads to large errors in estimating the hearing threshold.
In both cases, there is no need to measure exactly using only nominal stimulus frequencies. Deviations in the order of +100 Hz are tolerable for most applications. This is supported by the common DPOAE model, which predicts a certain region of the cochlea producing the DPOAE response. This region is thought to be close to the location tuned to f2, and covers a frequency range anyway.
In many applications, it is even desirable to cover a frequency band with a single DPOAE measurement instead of a single frequency, since often only a few tests at selected frequencies can be done in reasonable time, which are then used to characterize performance of hearing over the complete frequency range. Typically, test frequencies are spaced as coarse as octaves of half-octaves.
In order to overcome unwanted effects of the DPOAE fine structure, it is desirable to disable or attenuate the so-called second source. Methods have been suggested to mask the second source with additional stimulus tones, for example a tone that is close to fDPOAE.
However, these methods do not perform as stable as desired, and useful parameter settings vary strongly among subjects. The method described below provides a new approach for measuring DPOAE.